Fuel burner control systems, or systems that are commonly referred to as flame safeguard control systems, have been used for many years in nonresidential type burner control applications. These devices traditionally have been devices that operate through mechanical switches and relays. Since mechanical switches and relays provide "on-off" type of control or "go" or "no-go" functions the sensors used with the systems have been compatible switching type devices. These devices have been pressure operated, temperature operated, or flame operated. The sensor function would be to either provide an open or closed circuit.
This type of sensor structure has two significant faults. First, the sensor is incapable of providing ongoing information and is limited only to providing information as to a switched or limit condition. Secondly, this type of device is susceptible of being bypassed by users and maintenance people. Maintenance people traditionally jumper or open circuit sensors while troubleshooting. This type of troubleshooting can lead to serious and often unsafe conditions. Also, the ability to either short circuit or open circuit a sensor makes a system susceptible to being operated in an unsafe condition either intentionally or inadvertently by a person unaware of the risks involved.